FIGS. 18(a) and 18(b) are a cross-sectional view and a perspective view, respectively, illustrating process steps for producing a semiconductor substrate according to the prior art. The production method of the semiconductor substrate will be described with reference to these figures.
First, as shown in FIG. 18(a), an insulating film 1 comprising, silicon oxide is formed in a thickness of about 1 .mu.m on a semiconductor substrate 10 comprising, for example, monocrystalline silicon. A semiconductor film 2 comprising amorphous silicon or polycrystalline silicon is formed in a thickness of approximately 0.5 to 5 .mu.m on the insulating film 1. An insulating film 3 comprising a silicon oxide film and a silicon nitride film which are laminated is formed in a thickness of approximately 1 to 3 .mu.m on the semiconductor film 2. A semiconductor substrate material 500 is thus produced.
Next, as shown in FIG. 18(b), the semiconductor substrate material 500 obtained as described above is placed on a lower side heater 20a comprising carbon. An upper side strip heater 20b comprising carbon is disposed above the substrate material 500 a predetermined distance from the substrate material 500. The upper side strip heater 20b is scanned over the substrate material 500 from an end of the substrate material 500 to the opposite end at a predetermined speed of, for example, 1 mm/sec to zone melt and recrystallize the semiconductor thin film 2 in the substrate material 500. Then, in the semiconductor thin film 2, crystal grain size is increased by zone melting recrystallization, resulting in improved electrical characteristics such as improved electric conductivity of the semiconductor thin film 2. Here, an infrared lamp may be used in place of the carbon strip heater 20b.
FIGS. 19(a) and 19(b) are a cross-sectional view and a plan view, respectively, both illustrating the semiconductor substrate material 500 obtained by the above-described process. In the semiconductor thin film 2 in the semiconductor substrate material 500, crystal grain size is increased by zone melting and recrystallization, resulting in improved electrical characteristics such as electric conductivity. At the same time, however, some crystalline defects, such as sub-grain-boundaries 4 and dislocations 5, are generated in the semiconductor thin film 2 as shown in the figures. These sub-grain-boundaries 4 and dislocations 5 serve as cores for recombination of carriers, i.e., electrons and holes, after the device is produced, and these sub-grain-boundaries and dislocations cause an increase in a dark current of a light-responsive device. Especially, because sub-grain-boundary 4 is an aggregate of dislocations 5, in order to improve the device characteristics, it is important to reduce sub-grain-boundaries 4 and to produce sub-grain-boundaries 4 at positions where sub-grain-boundaries 4 do not aversely affect device characteristics. Here, sub-grain-boundary 4 is supposed to be caused at the solid-liquid phase interface which is generated in the semiconductor thin film 2 during zone melting and recrystallization.
FIGS. 20(a) and 20(b) are cross-sectional views illustrating conventional process steps for producing a semiconductor substrate which is improved so as to control the positions of sub-grain-boundaries in view of the above-described problems, as disclosed in "Journal of Electrochemical Society, Vol. 130, No.5, pages 1178 to 1183". In the figures, the same reference numerals as those in FIGS. 18(a) to 19(b) designate the same or corresponding elements, and reference numeral 20 designates carbon stripes of about 7 .mu.m thickness. The process steps will be described below.
First, as shown in FIG. 20(a), similar to the process step shown in FIG. 18(a), an insulating film 1 comprising a silicon oxide film, a semiconductor film 2 comprising amorphous silicon or polycrystalline silicon, and an insulating film 3 comprising a silicon oxide film and a silicon nitride film which are laminated are successively deposited in this order on a substrate 10 comprising monocrystalline silicon. Carbon stripes 20 of 7 .mu.m thickness are periodically disposed on film 3 with a predetermined interval of approximately 100 to 200 .mu.m to provide a semiconductor substrate material 500a.
Next, similar to the process step shown in FIG. 18(b), when zone melting and recrystallization of the semiconductor film 2 is carried out along the direction of the carbon stripes 20 using a carbon strip heater, the heat from the carbon strip heater is absorbed by the carbon stripes 20, so that a significant amount of heat is transfered to the portions of the semiconductor film 2 located below the carbon stripes 20. Therefore, a temperature distribution according to the positions of the carbon stripes 20 is formed in the semiconductor film 2, the solid-liquid phase interface is modulated by the temperature distribution, and sub-grain-boundaries 4 are produced in the semiconductor film 2 only below the carbon stripes 20. In this way, not only sub-grain-boundaries 4 can be controlled to particular positions but also the number of sub-grain-boundaries 4 can be reduced. The reason why the carbon stripes 20 are formed with intervals of approximately 100 to 200 .mu.m is because if the interval is larger than 200 .mu.m, sub-grain-boundaries 4 are unfavorably generated at portions of the semiconductor film 2 other than below the carbon stripes 20. This method cannot reduce the number of dislocations.
FIGS. 21(a) and 21(b) are cross-sectional views illustrating process steps for producing a semiconductor substrate in accordance with "Extended Abstracts of the 18th (1986 International) Conference on Solid State Devices and Materials, Tokyo, 1986, pages 565 to 568". In the figures, the same reference numerals as those in FIGS. 18(a) to 20(b) designate the same or corresponding elements. The process steps will be described below.
In the process steps, as shown in FIG. 21(a), a predetermined region in a substrate 10 comprising monocrystalline silicon is oxidized to form an insulated region 1a. A semiconductor thin film 2 comprising amorphous silicon or polycrystalline silicon is formed on the substrate 10 comprising monocrystalline silicon. A silicon nitride film 3a having thick stripe portions 3b arranged periodically at a predetermined interval is formed on the semiconductor thin film 2 to provide a semiconductor substrate material 500b. Then, the thickness of the thin portion of the silicon nitride film 3a is, for example, 60 .ANG., and the thickness of the thick stripe portion 3b of the silicon nitride film 3a is 550 .ANG..
Next, laser light from a laser light source (not shown) irradiates the surface of the silicon nitride film 3a and the laser light is scanned at a predetermined speed along the stripe direction of the thick stripe portion 3b to carry out the zone melting recrystallization of the semiconductor thin film 2. During the zone melting recrystallization, as shown in FIG. 21(b), since the thick stripe portions 3b of the silicon nitride film 3a are heated more than other portions, more heat is applied to the portions of the semiconductor film 2 below the stripe portions 3b, so that sub-grain-boundaries 4 are produced at the portions of those semiconductor film 2. As for the zone melting and recrystallization, a portion 10a of the substrate 10 comprising monocrystalline silicon, contacting the semiconductor film 2, acts as a seed, and the zone melting recrystallization proceeds along a crystalline direction of the substrate 10 comprising monocrystalline silicon including no defects, resulting in reduced dislocations 5 as compared with the case where the zone melting recrystallization is carried out as shown in FIGS. 20(a) and 20(b), whereby the defect density is reduced.
As described above, in the prior art process steps for producing a semiconductor substrate, in the zone melting and recrystallization of a semiconductor film, carbon stripes of predetermined width are periodically disposed on the upper surface of an insulating film covering the semiconductor film, or the semiconductor film is covered with an insulating film having thick stripe-shaped portions periodically arranged on the insulating film, whereby the number of sub-grain-boundaries generated in the semiconductor film is reduced, and adverse effects on the device characteristics by sub-grain-boundaries after a semiconductor device is actually produced are reduced to a great extent by controlling the positions of sub-grain-bounderies.
However, as long as sub-grain-boundaries exist in the semiconductor film, deterioration of device characteristics cannot be completely avoided. Accordingly, in order to further improve device characteristics, it is necessary to completely remove sub-grain-boundaries from the recrystallized semiconductor film or to inactivate sub-grain-boundaries in the semiconductor film. In order to realize this, it is thought to carry out a required patterning of the semiconductor substrate obtained by the process steps shown in FIG. 20 or 21 to remove or inactivate the sub-grain-boundary portions. In the process steps shown in FIGS. 20(a) to 20(b), however, after removing carbon stripes 20 or insulating film 3a comprising a silicon nitride film, a further mask pattern is required to be produced to remove or inactivate the sub-grain-boundary portions. When process steps for forming the mask pattern and removing the same are added, the process for producing the semiconductor substrate as a whole is complicated. In addition, if the positional accuracy in producing the mask pattern is reduced, complete removal of sub-grain-boundaries is disabled.
Furthermore, in the above-described prior art process steps for producing a semiconductor substrate, in order to for zone melting and recrystallization of a semiconductor film uniformly in the semiconductor film to occur, the thickness of the semiconductor film is limited to approximately 5 .mu.m which is described above. Accordingly, when a semiconductor film of more than several .mu.m thickness, more concretely, approximately 50 to 100 .mu.m, which is used in such as a solar cell, is formed, there are problems that not only crystalline grain size in the semiconductor film cannot be increased, but also defects are increased.